The present invention relates to cellular telecommunications, and more particularly to methods and apparatuses that enable a user equipment (UE) in a cellular telecommunications system to distinguish between synchronized and unsynchronized network operation and additionally to adapt its operation accordingly.
The forthcoming Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) Long Term Evolution (LTE) technology, as defined by 3GPP TR 36.201, “Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description” will be able to operate over a very wide span of operating bandwidths and also carrier frequencies. Furthermore E-UTRAN systems will be capable of operating within a large range of distances, from microcells (i.e., cells served by low power base stations that cover a limited area, such as a shopping center or other building accessible to the public) up to macrocells having a range that extends up to 100 km. In order to handle the different radio conditions that may occur in the different applications, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink (i.e., the communications link from the base station to UE) because it is a radio access technology that can adapt very well to different propagation conditions. In OFDMA, the available data stream is portioned out into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband it only experiences flat-fading. This makes it very easy to demodulate each subcarrier at the receiver.
Furthermore LTE technology operates in both synchronized and asynchronous networks. In a synchronized network, all of the base stations (e.g., eNodeBs) use the same timing over the air interface, whereas in an asynchronous network, a base station's air interface timing could differ from its neighbor's. The radio channel properties and characteristics of the received signals vary, depending on whether the network is synchronized or not. To take one example, in the case of synchronized networks, the estimated channel in the downlink is typically a multi-channel estimate of all contributing radio base stations. The channel estimate is degraded because the reference signals collide with one another. By contrast, in an asynchronous network the reference signals collide with data rather than with other cells' reference signals, thereby giving a more random behavior that can be treated as noise.
To take another example, the characteristics of the received signal will also differ, depending on whether the network is synchronized or unsynchronized. One example is the synchronization channel, whose signals enable a UE to synchronize its own operation with that of its serving cell: Even though different synchronization signals are used in the different cells, their occurrence in time will coincide in a synchronized network, whereas this is much less likely in an asynchronous network. Therefore, a receiver optimized for use in a synchronized network will not treat another base station's synchronization signals as interference, but rather as a competing synchronization signal.
These two examples show that it is beneficial for the receiver to know whether it is operating in a synchronized or asynchronous network.
Conventional UEs do not detect the synchronization state of a network. Consequently, the UE's algorithms and procedures must be designed to operate satisfactorily in both synchronized and asynchronous networks. This compromise results in sub-optimal performance in both cases. If the UE knew the synchronization state of the network in which it was operating, it would be able to perform the best algorithms (e.g., cell search and channel estimation) under the circumstances.
It is therefore desirable to provide a mechanism that enables a UE to determine whether the network in which it is operating is synchronized or not in order to allow it to select best suited algorithms/procedures for subsequent operations.